How does secondary xylem become wood in a tree trunk?

In woody plants, wood is formed through the activity of the vascular cambium, a lateral meristem that develops between the primary xylem and primary phloem. This cambium functions as a cylinder of dividing cells that produces new tissues: it generates secondary phloem toward the outside of the stem and secondary xylem—commonly known as wood—toward the inside.12

The vascular cambium consists of two cell types:
* Fusiform initials: Elongated cells that produce the vertical system of the secondary xylem, including tracheary elements and fibers.12
* Ray initials: Squat cells that produce the horizontal vascular rays, which facilitate the storage and translocation of water and nutrients.12

As the cambium produces more cells internally than externally, the secondary xylem accumulates, causing the tree trunk to thicken. Over time, these cells are reinforced by lignin, a complex polymer that provides the structural stiffness and compressive strength characteristic of wood. As the stem grows in circumference, the oldest xylem layers become the central heartwood, while the younger, outer layers remain functional as sapwood.123